1. Field of the Invention
This invention relates to an improved process for converting a methanol feed to light olefins over modified crystalline aluminosilicate zeolite catalysts.
2. Description of the Prior Art
A remarkable growth in the production of synthetic fibers, plastics and rubber has taken place in recent decades. Such growth, to a large extent, has been supported and encouraged by an expanding supply of inexpensive petroleum raw materials such as ethylene and propylene. However, increasing demand for these light olefins has, from time to toime, led to periods of shortage, either due to a diminished supply of suitable feedstocks or to limited processing capacity. In any event, it is now considered highly desirable to provide efficient means for converting raw materials other than petroleum to light olefins.
One such non-petroleum source of light olefins is coal-derived methanol. In this respect, it is known that methanol can be catalytically converted to olefin-containing hydrocarbon mixtures by contact under certain conditions with particular types of crystalline zeolite catalysts materials. U.S. Pat. No. 4,025,575, issued May 24, 1977, to Chang et al and U.S. Pat. No. 4,083,889, issued Apr. 11, 1978 to Ceaser et al, for example, both disclose processes whereby methanol and/or methyl ether can be converted to an olefin-containing product over a ZSM-5 type (Constraint Index 1-12) zeolite catalyst. ZSM-5, in fact, converts methanol and/or methyl ether to hydrocarbons containing a relatively high concentration of light (C.sub.2 and C.sub.3) olefins.
Modification of zeolite methanol conversion catalysts with, for instance, silica, phosphorus, metal ions or metal oxides, can enhance selectively of the methanol conversion reaction for production of light olefins. For example, Wunder et al.; U.S. Pat. No. 4,247,731; Issued Jan. 27, 1981 and Wunder et al.; U.S. Pat. No. 4,296,266; Issued Oct. 20, 1981, both disclose modification of zeolites such as Zeolite X with manganese and optionally with additional materials such as magnesium in order to enhance ethylene selectively of such zeolites when they are used to catalyze methanol/dimethyl ether conversion.
It is also known that light olefin, and especially ethylene, production from the catalytic conversion of methanol can be optimized by varying one or more reaction parameters. Variations in reaction temperature, pressure and reactant space velocity can, for example, alter the selectivity of the methanol conversion reaction to produce different kinds of hydrocarbon products, e.g. to maximize light olefin production.
Likewise, utilization of dilute methanol feeds or inert diluents can also tend to increase selectivity of the reaction toward ethylene and light olefin production. Notwithstanding the existence of process suitable for converting methanol to high yields of light olefins, there is a continuing need to develop additional catalytic procedures suitable for converting an organic charge comprising methanol to light olefin products with improved light olefin selectivity and especially with improved selectivity to propylene, which, like ethylene, is a valuable feedstock material.
Accordingly, it is an object of the present invention to provide an improved process for converting a methanol feed to olefin-containing products with high selectivity to production of propylene.
It is a further object of the present invention to provide such a selective methanol conversion process which can be employed in conjunction with known catalysts and processes for maximizing light olefin product from methanol.
It is a further object of the present invention to provide such a selective methanol conversion process employing conventional catalysts, readily available reactants and diluents and commercially practical reaction conditions.